With the increasing bandwidth demands from the advent of the Internet, service providers have looked for ways to increase data performance over the copper wire twisted-pair local loop transmission lines that connect the telephone central offices (COs) to the customer premises (CPs). The customer premises equipment (CPE) is connected to the CO switches over transmission lines known as “local loops,” “subscriber loops,” “loops,” or the “last mile” of the telephone network. Historically, the public switched telephone network (PSTN) evolved with subscriber loops connected to a telephone network with circuit-switched capabilities that were designed to carry analog voice communications. Digital service provision to the customer premises is a more recent development, with the evolution of the telephone network from a system just designed to carry analog voice communications into a system which could simultaneously carry voice and digital data.
Because of the prohibitive costs of replacing or supplementing existing subscriber loops, technologies have been implemented that utilize existing subscriber loops to provide easy and low cost customer migration to digital technologies. Subscriber loops capable of carrying digital channels are known as digital subscriber lines (DSLs). Logical channels within a subscriber line which carry digital signals are known as DSL channels, while logical channels within a subscriber line which carry plain old telephone service (POTS) analog signals are known as POTS channels. Furthermore, to provide customers with additional flexibility and enhanced services, frequency-division multiplexing and/or time-division multiplexing techniques have been designed to fill a subscriber loop with multiple logical channels. These newer DSL technologies provide digital service to the customer premises without significantly interfering with the existing POTS equipment and wiring. The newer DSL technologies accomplish this functionality by frequency-division multiplexing (FDM) their digital signal above (at higher frequencies than) the 0 KHz to 4 KHz baseband of standard, analog POTS signals. Multiplexing techniques and terminology are common to those skilled in the art, and are not described herein.
Several variants of new DSL technology exist (e.g., ADSL, SDSL, RADSL, VADSL, MVL™, Tripleplay™, etc.), with this group generally referred to as xDSL. Communications systems carrying xDSL usually multiplex xDSL signals and a POTS signal onto a single physical local loop.
Historically, the POTS subscriber loop was designed with the functions needed to communicate both analog, voice-conversation signals and subscriber loop signaling. The CO switch uses subscriber loop signaling to notify the customer premises about events in the telephone network, while customer premises equipment (CPE) use subscriber loop signaling to inform the CO to perform actions for the customer. Some examples of subscriber loop signaling include: the CO switch signaling to the CPE that an incoming call has arrived by ringing the phone, the CPE (e.g., a telephone) signaling to the CO switch that the CPE is initiating a call by an on-hook to off-hook transition of the telephone handset, and the CPE signaling to the CO switch that a call should be connected to a location by sending the phone number of the location.
Although the transmission of both digital and analog POTS signals over a subscriber loop offers many potential advantages for customers, several practical problems must be solved in implementing DSL solutions. One significant problem resulting from the POTS subscriber loop signaling functions is the generation of high-frequency interference, known in the art as noise, into DSL channels. For instance, the on-hook/off-hook signal and the pulse-dialing signal are square waveforms which have high-frequency components and harmonics, and theoretically require infinite frequency bandwidth. This high-frequency noise may degrade the signal to noise (S/N) ratio of the DSL channel. The S/N ratio is commonly known to those skilled in the art, but can be simply described as the ratio of the transmit signal amplitude to the noise amplitude, expressed in decibels (dB). Thus, a heretofore unaddressed need exists in the industry for a way to prevent or substantially minimize the adverse affects on the DSL channel S/N ratio caused by noise introduced by the POTS subscriber loop functions.
Another practical problem facing the industry effort to implement DSL technology on the existing PSTN system is the large voltage magnitude change occurring on the subscriber loop during transitions between on-hook and off-hook conditions, as is well known in the art. Some embodiments of prior art DSL technology require a change in the input impedance of the DSL device upon sensing of a transition between on-hook and off-hook conditions. Thus, a heretofore unaddressed need exists in the industry for a way to prevent or substantially minimize the adverse affects of the on-hook/off-hook transition.
Another practical problem facing the industry effort to implement DSL technology on the existing PSTN system is the unpredictable nature of the subscriber loop transmission system impedance. Signal attenuation (decrease in signal strength) and signal distortion (changes in the signal shape) are caused by real and reactive impedance losses incurred on the subscriber loop as the signal is transmitted between the CO and the CPE. Each subscriber loop, consisting of a copper wire twisted-pair circuit connecting the CO to the CPE, is unique. That is, each subscriber loop differs in length, and often these subscriber loops are constructed with varying copper wire gauge sizes. Therefore, the actual circuit impedance of any given subscriber loop is unique and different from other subscriber loops. DSL technology utilizes FDM to shift the frequency of the communication signal into the 25 KHz to 1 MHz frequency range. As is well known in the art, subscriber loop circuit impedance is not a constant, but rather a variable over the frequency spectrum because the subscriber loop impedance is complex (having reactive impedance components as well as resistive impedance components). Therefore, signal attenuation also varies with the frequency of a transmission signal. That is, some frequencies will be attenuated more or less than other frequencies.
The presence of bridged taps connected to the subscriber loop introduces another unpredictable impedance component. Bridged taps are unused copper wire twisted-pair lengths connected at various points of the subscriber loop. Bridged taps constitute parallel circuits which alter the impedance of the subscriber loop circuit, and effectively reduce the transmit signal strength.
Finally, the wiring of the customer premise and the various types of customer equipment and devices, including multipoint communication, connected to the subscriber loop is unique. These differences at the customer premise also impact the overall impedance of the subscriber loop transmission system.
For the purpose of establishing the transmitter frequency domain specifications and limits, current practice typically models the subscriber loop impedance as a resister, RL, that is representative of the characteristic impedance of the subscriber loop transmission line. At the remote end of the transmission line, the receiver equipment is typically modeled as a terminating resister, RR, usually of the same value as RL. Transmission of signals onto subscriber loops has been provided by a voltage signal source, Vs, and a series resister, RT. Current practice is to transmit at the subscriber loop transmission line input a transmit signal spectral shape of VS that is designed to be the same as a voltage power spectral distribution (PSD) standard. The PSD standard specifies maximum signal strength (amplitude) and frequency bandwidth boundaries for a DSL channel.
Design of the transmit signal spectral shape of VS necessarily requires certain assumptions about the subscriber loop transmission system. Traditional transmission line theory teaches that for optimum communication, the subscriber loop transmission system should have RT=RL=RR. As an example, it is customary in some DSL technologies to select RL=135 ohms for transmission signals in the band from approximately DC to 192 kHz. This 135 ohm value is a reasonable best choice for a simplistic resistive compromise model. Because the prior art model is resistive, the design transmit signal is the same as the design PSD of VS.
However, the prior art assumptions may be wholly inadequate in representing the wide range of subscriber loop transmission lines found in practice. RT is not ideal (RT≠RL≠RR) since each individual subscriber loop is unique. Also, RL is not ideal because customer premises wiring are often different and because of bridged taps on the subscriber loop. In practice, the actual subscriber loop transmission system impedance can vary in magnitude from well over 200 ohms to less than 50 ohms, and the actual impedance is complex. The result in practice is that the actual transmit signal on any given transmission line can vary dramatically, and this variance is usually such that the transmit signal amplitude is lower than permitted in part of or all of the transmission band as defined by the PSD standard. It can be shown, for example, that the actual transmit signal amplitude can be 12 dB lower than the PSD standard in part of the band, and even average power can be 6 dB lower than allowed. This means that 6 dB or more of potential transmit signal power is being sacrificed, and that the receive signal S/N ratio is thus 6 dB lower than the S/N that could be realized with an optimized transmit signal.
Another problem involves instances where the actual transmit signal voltage exceeds the PSD standard. If the actual transmit signal voltage exceeds the PSD standard, undesirable interference or noise is induced onto other subscriber loops sharing the same underground cable or overhead wire.
Thus, a heretofore unaddressed need exists in the industry for a way to provide for a transmit signal which conforms to a defined PSD standard regardless of the actual impedance characteristics of the transmission system.